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I am remodelling a bathroom and completely gutted it. I have 3/4 inch subfloor down and will be putting 1/2 underlayment then tile. When is the best time to put up drywall on the walls. Should I put up drywall before or after the underlayment or does it matter? What I've read is to keep the drywall 1/4 inch up off the floor but don't know if that is subfloor, underlayment, or finished floor. Thanks.

the underlay should go down after the drywall is complete.. by doing it later the fresh plywood will have less chance of getting covered in gobs of dropped drywall mud which wil have to be sc****d off before the finish floor goes down

the underlay should go down after the drywall is complete.. by doing it later the fresh plywood will have less chance of getting covered in gobs of dropped drywall mud which wil have to be sc****d off before the finish floor goes down

Not only would it have to be scr aped, you run the risk of it swelling in the areas where gobs of drywall mud hit the floor. Not an overly big deal, but since the subfloor's purpose is to provide a baby's butt smooth and clean surface, why F it up? Put up the drywall and leave the 1/2" gap above the finished height of the underlayment. Tile/flooring can butt to the drywall, base will cover any gap thereafter.

The underlayment should go down after the deflection of the floor joists is checked to make sure the framing can support tile. You'll need a deflection of L360 for ceramic and porcelain, L720 for natural stone. Natural stone also gets 1.25" of underlayment.

You can get a ton more information on the John Bridge Tile forum. In the dark blue bar there is a link to the DEFLECTO meter which can calculate. The library also has more links.

L360 is a measurement of the linear deflection over a given span.

Ditra and CBU both have no effect on delfection. The joists have deflection to calculate along their axis, and the plywood has deflection between the joists. To handle the delfection between the joists a bare minimum 5/8" plywood (NO CDX or sheathing grade) with 16" OC joist spacing.